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“Individual variation is the raw material of evolution,” says Indiana University’s Kimberly Rosvall. “We report that free-living birds vary in aggression and the more aggressive individuals express higher levels of genes related to testosterone processing in the brain.” Rosvall’s study of wild junco birds demonstrates that individual variation in brain sensitivity to hormones, not the actual amount of hormone present, correlates with behavior. The study also uncovers a mechanism by which hormones like testosterone promote aggression. The researchers believe their results help explain the evolution of aggressive behavior.

The well-known fact that hormones affect behavior is the basis, of course, for such practices as the gelding of animals. “But very few people have looked to see if individuals actually do vary in expression of these genes [related to hormonal sensitivity], and whether this individual variation means anything, in terms of an animal’s behavior,” Rosvall says. “Our work shows that it does.”

Hormones, like testosterone and estrogen, are chemical messengers. They circulate throughout the body in the blood and therefore can affect many target organs, including the brain. The cells of target organs must have receptors for a particular hormone in order to react to it. Those receptors are manufactured when genes directing their construction are expressed. Therefore, if high levels of messenger RNA (mRNA) associated with a particular gene are found, that gene is strongly expressed.

Dr. Rosvall’s team found that wild junco birds with high levels of mRNA for several hormones demonstrated more aggressive behavior toward birds of the same sex.

Dr. Rosvall’s team found that wild junco birds with high levels of mRNA for several hormones demonstrated more aggressive behavior toward birds of the same sex. Elevated sensitivity to the hormones was present in areas of the birds’ brains associated with aggressive behavior and song control. Both sexes demonstrated predictable behaviors such as flyovers, dive-bombing, and territorial singing. For example, males with more mRNA for estrogen receptors sang more songs at intruders. Dive-bombers of both sexes had more testosterone-related mRNA.

Even though hormones are known to affect behavior, circulating levels of the hormone testosterone do not correlate well with aggression. From this study, for junco birds at least, it seems that genetically mediated brain sensitivity to testosterone influences aggressive behavior, rather than the amount of testosterone present.

“On the one hand, we have lots of evidence to suggest that testosterone is important in the evolution of all kinds of traits,” Rosvall explains. “On the other hand, we know that individual variation is a requirement for natural selection, but individual variation in testosterone does not always predict behavior. This conundrum has led to debate among researchers about how hormone-mediated traits evolve.”

Finding this strong relationship between individual genetic expression and a behavior that affects reproductive success prompts evolutionary scientists to claim discovery of a mechanism by which “evolution could shape behavior via changes in the expression of these genes.”

Why these genes are expressed more strongly in certain birds remains an open question. There is a possibility, researchers note, that environmental pressures could up-regulate or down-regulate such genetic expression. Similar epigenetic controls have been found in other areas of animal and human biology. While that idea has a somewhat Lamarkian ring to it, the increase of various traits within a created kind or even a species has nothing to do with evolution of new kinds of animals. Environmentally influenced genetic expression—if that turns out to be the case here—would only help explain how behavioral variations occur within a created kind.

The findings in this study, whatever the cause of variation, help explain how behavioral variations can occur within a kind. However, change within a created kind is not evidence for evolution of one kind into a new kind. Such “new kinds” would require new genetic information, not just an altered expression of existing information. Evolutionists have never provided a valid biological mechanism for producing new genetic information.

No evolution in the molecules-to-man sense is required to produce an angry bird. And neural sensitivity to hormones is quite inadequate to even begin to produce a non-bird. Birds, like all kinds of animals, were created to reproduce after their kinds and given the ability to change within those kinds. Studies like this show us one of the ways such variation can occur, even producing more aggressive varieties and species.

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